This invention relates to a process for the production of friction materials. More particularly, it relates to a process for producing high-performance friction materials that are markedly improved in friction characteristics, wear resistance, heat resistance and anti-fade quality so that they are particularly suitable for use in brakes and clutches.
Friction materials for use in brakes, clutches, etc. are conventionally produced by binding a friction modifier, a filler and a reinforcement with a thermosetting resin such as a phenolic resin, and molding the blend.
Friction materials, in particular those for use in brakes, are required to exhibit extremely high performance in terms of friction characteristics, wear resistance, heat resistance and anti-fade quality and the greater part of their performance is determined by the performance of the binder resin. With conventional friction materials which use phenolic resins and other thermosetting resins as binders, it is impossible to completely eliminate the problems that result from the deterioration of binders with time and their low heat resistance, i.e. the change in friction characteristics, wear, thermal crack and fade. Under the circumstances, studies have been made of using other thermosetting resins as binders (e.g. furan resins and modified phenolic resins) but, as of today, no friction materials have yet been produced that far excel the prior art versions in performance.
Another class of friction materials that have been the subject of extensive studies are those which use sintered metals or carbon/carbon composites unlike the conventional friction materials which use thermosetting resins as binders. However, sintered metals involve serious problems such as "vapor lock" due to their high heat conductivity and fusion of the friction surface under high load, whereas carbon/carbon composite materials suffer from the problem that they exhibit only instable friction characteristics and low frictional resistance during low-and medium-speed running due to their inherent sliding quality and that they will wear rapidly. Further, the two kinds of friction materials have a common disadvantage in that their production rate is so low that they are very expensive. As a result, friction materials made of sintered metals or carbon/carbon composite materials find very limited use.
With a view to solving these problems of the prior art, the present inventors previously studied a friction material using mesophase pitch as a binder and developed a friction material of very high performance that was capable of exhibiting consistent friction performance even under high-load conditions (Japanese Patent Public Disclosure No. 219924/1988). However, to produce that friction material, hot molding under high-temperature conditions of 400.degree.-650.degree. C. and, hence, an expensive press unit are necessary, which is an obstacle to the implementation of commercial production.